Conventional scanning microscopes comprise essentially a focusing means to focus radiation from a light source onto an object to be inspected, a radiation detector, and scanning means to cause relative movement between the point of focus and the object.
The ultimate limit of resolution of state of the art optical devices, such as microscopes, is of the order of one wavelength, i.e., about 500 nm. Two neighboring object points are considered to be just resolved if in the image the principal diffraction maximum of the first object point coincides with the first diffraction minimum of the second object point (Lord Rayleigh, Phil. Mag. (5), 8(1879) 261).
The distance Y between two object points which an optical microscope can just resolve, when the illumination is incoherent and the microscope's aperture is circular, is .about.0.61.lambda./n.multidot.sin.theta., wherein the term n.multidot.sin.theta. is the "numerical aperture", i.e. the product of the refractive index n and the size of the semi-angle .theta. of the cone of rays in the object space. The numerical aperture should be large if a high resolving power is to be achieved (M. Born and E. Wolf, Principles of Optics, Pergamon Press, London 1959, p. 417(f). Considering that the largest numerical apertures so far achievable are about 1.3 . . . 1.4, the best resolution is of the order of 0.5.lambda..
The above-mentioned resolution limit is derived under the assumption that the optical instrument is based on imaging and implies that both the diameter 2a of the entrance pupil of the objective and its distance h from the object are large compared to the wavelength .lambda. of the illumination used (a, h&gt;&gt;.lambda.). Because of the shortness of the wavelengths present in visible light compared with the smallest diameter to which an entrance pupil could be manufactured hitherto, this condition is satisfied in conventional optical instruments in a natural way.
Numerous attempts to increase the resolving power of microscopes are known from the prior art. In U.S. Pat. No. 3,926,500 a diaphragm having small openings is rotated in a plane conjugate to the object plane. The object to be inspected is illuminated through the diaphragm such that light passing through its openings is sharply focussed only on areas lying in or near to the object plane within the depth of focus range of the objective. Accordingly, only light reflected from said areas can contribute to the formation of a clear image. When either the object or the diaphragm are cyclically shifted in the direction of the optical axis, the depth of focus of the microscope can be somewhat extended, with the disadvantage, however, that the actual roughness of the surface inspected is equalled out.
According to the literature reference "Optische Abbildung unter Ueberschreitung der beugungsbedington Auflosungsgrenze" by W. Lukosz and M. Marchand, Optics Acta 10 (1963) p. 241, the resolution of the optical system can be increased by a grid-like arrangement of the scanning diaphragm pinholes.
U.S. Pat. No. 4,198,571 describes the improvement of the resolution of scanning microscopes through the use of an annular lens, which may be a circular lens with a closely spaced annular aperture. A disadvantage with such an arrangement is the severe loss of power through the annulus which requires the use of a sufficiently powerful source of coherent light, such as a laser.
The references cited clearly indicate that efforts have been made to increase resolution by pushing the natural limitation given by the dimensions of the optical elements, particularly lenses, used in optical microscopes. None of the references proposes a microscope having only a submicron optical aperture without imaging elements.
The present invention is an optical microscope which circumvents the above-described resolution limit through the use of an aperture with an entrance pupil diameter 2a and a distance h from the object which are small compared to the wavelength of the light. The aperture receives a signal, the intensity of which depends on the transmissivity of a spot on the illuminated object directly opposite its entrance pupil. When scanned along the surface, the intensity varies according to the objective transmissivity. The record of the scan represents an "image" of the object. The resolution of the "image" can be substantially below the classical resolution limit, say .lambda./10.
A key element of such an optical microscope is, of course, the aperture, and the following specification will in part be devoted to a description of an aperture manufactured from the a pyramid-shaped transparent crystal, the apex of which has been machined to yield a radius of curvature equal to or less than the desired resolution.
A pyramid-shaped lens is known from the article "Self-Image and Enlarging Lens" by T. S. Fitzgerald in IBM Technical Disclosure Bulletin, Vol. 18 (1976), p. 4174. This lens is used for enlarging an image recorded on photographic film (e.g. microfiche) and for displaying it on its frosted base surface. A lens as disclosed in this reference, apart from having macroscopic dimensions, cannot be used in a microscope application as more is needed than just cutting the apex: provision must be made to precisely delineate the borders of the aperture, a measurement not required in the macroscopic application shown in the reference.
Accordingly, it is a primary object of the present invention to provide a new optical microscope having improved resolution.
It is another object of the present invention to provide a scanning optical microscope having an improved optical aperture.
It is a further object of the present invention to provide a scanning optical microscope having an aperture with an entrance pupil of submicron dimensions.
It is a still further object of this invention to provide a scanning optical microscope whose resolution limit can be less than the classical resolution limit of the wavelength of the light used in the microscope.